A model describing the concentration and temperature dependences of gas and vapor permeability in rubbery polymers is presented. The solubility and permeability of propane in poly(dimethylsiloxane) (PDMS), a commercially used vapor separation membrane material, were determined over a wide range of temperatures and pressures to test the model. The model describes propane permeability in PDMS with an average error of 8.2%. The model also accurately predicts a decrease in propane permeability in PDMS with decreasing permeate pressure at fixed feed pressure. The model has also been tested satisfactorily using literature data for transport of condensable penetrants in PDMS and poly(ethylene).